Two teenagers looking for a little fun in a quiet northern California suburb have been accused of getting around a late-night internet ban by lacing milkshake with ground-up sleep medication and serving it to one of the girl's parents.

The mother and father of a 16-year-old girl in Rocklin, California, were in the habit of shutting off the home internet at 10pm. In order to get round the curfew, their daughter and a 15-year-old friend offered to pick up shakes from a fast-food restaurant, which they then allegedly drugged.

According to police, the parents thought the shakes had an odd taste and stopped drinking them. An hour later, the parents were asleep, and the girls were able to use the internet as they pleased.

The parents reported hangover-like symptoms when waking up at about 1am, and again in the morning. Unsure why, they bought a $5 drug testing kit at the local police station to test themselves. When the results came up positive, they returned to the police station with their daughter.

Lieutenant Lon Milka of the Rocklin police department told the Guardian that detectives were still investigating how much medication had been used in the shake. He said the medication had been provided by the visiting friend, from her home in the neighboring town of Roseville.

California laws limit how much information law enforcement officers can provide on juvenile cases, but Milka said of the parents: "They developed enough information in order to bring their daughter down here."

The girls have been charged with willfully mingling a pharmaceutical into food and conspiracy. The were booked in Placer County juvenile hall on New Year's Eve. Police have not disclosed what the girls were using the internet for.

Most juvenile crimes in the town of over 58,000 people are minor offenses like vandalism, alcohol and drug use. "Nothing where they are endangering or mingling the pharmaceuticals with the milkshake – that is way out of line," Milka said.